[Fix] Fix DCN fp16 (#1014)

* [Fix] Fix DCN fp16

* add comment

mmcv/ops/deform_conv.py

@@ -70,6 +70,9 @@ class DeformConv2dFunction(Function):
         ctx.deform_groups = deform_groups
         ctx.im2col_step = im2col_step
 
+        # until the code is modified for torch.cuda.amp.autocast,
+        # we need to cast weight to avoid type mismatch in fp16 training
+        weight = weight.type_as(input)
         ctx.save_for_backward(input, offset, weight)
 
         output = input.new_empty(